DIVING SPEED AND FOOD-SIZE SELECTION IN COMMON GUILLEMOTS, URIA-AALGE

Results of choice experiments showed that guillemots Uria aalge prefer sprats Sprattus sprattus of a body depth of 16 to 17 mm. This is notably smaller than the maximum size that guillemots are able to swallow (body depth about 44 mm). A small proportion of the selected fish was used for display on the breeding ledge; these were significantly bigger than those preferred for consumption. Mean velocity during horizontal underwater flight was 2.18 m.s-1, with the maximum recorded being 2.57 m.s-1. This is considerably faster than the speed of any prey.